1. Field of the Invention
The present invention relates generally to Hall sensors and in particular to the arrangement and control of several Hall sensor elements in a Hall sensor array for magnetic field measurement with offset compensation.
2. Description of Prior Art
An individual Hall sensor element generally consists of an n-doped active semiconductor region on a p-doped semiconductor substrate. The n-doped active region is normally connected to an external control logic via four contact electrodes or contact terminals which are arranged diagonally opposite one another in the active region. The four contact electrodes of the Hall sensor element subdivide into two facing control current contact electrodes, which serve to generate a current flow through the active region, and two facing voltage tapping contact electrodes, which serve to tap a Hall voltage, which arises when a magnetic field is applied at right angles to the current flow in the active region, to act as a sensor signal.
A Hall sensor array in which two or four Hall sensor elements are used to compensate the disturbing effect of a particular crystal direction is know frown the European patent specification EP-0548391 B1. The angular separation of the individual Hall sensor elements is fixed, lying between 0xc2x0 and 180xc2x0. The angle is chosen according to the crystal direction of the semiconductor material which is used. According to EP-0548391 each Hall element is fed from a separate current source, so that a constant current is impressed on each element. The Hall voltages tapped off at the individual Hall elements in the Hall detector are connected in parallel in a switching stage. A common value is thus imposed on the Hall voltages of the individual elements, so that compensating currents may result.
As is known, inhomogeneities or faults in the semiconductor material of the active region often arise in the manufacturing processes of semiconductor structures due to the nature of production. Even with very refined manufacturing methods, these inhomogeneities cannot be avoided completely. These inhomogeneities are often the cause of an offset of the sensor signal. This means that a sensor signal is detected at the contact electrodes at which the Hall voltage is tapped off even when no magnetic field is being applied to the active region. This disturbing sensor signal is termed the offset of the useful sensor signal or simply the offset signal. Owing to the strong dependence of the offset signal on the inhomogeneities, traditional Hall sensor elements are subject to considerable scatter from one element to another. In addition there is a marked adverse effect or the sensitivity and measurement accuracy of the Hall sensors. For this reason, offset compensation and the correct evaluation or the sensor signals generally entail a costly circuit investment.
So-called piezo effects, which are strongly dependent on the crystal direction of the semiconductor material used, constitute another problem area in Hall sensor arrays. These piezo effects can produce a considerable offset signal through mechanical stresses caused by external forces (e.g. due to the housing) or through mechanical stresses in the crystal lattice of the semiconductor material. Attempts have been made to overcome these problems, either by matching the Hall sensor array to the crystal direction of the semiconductor material or by compensating the piezo effects through a suitable choice of the current directions in the semiconductor material depending on the crystal direction. However, these measures involve very complicated manufacturing processes for the Hall sensor arrays since it is necessary to take into account both the crystal orientation of the semiconductor surface and also the orientation of the Hall sensor elements to one another and in relation to the crystal orientation.
It is the object of the present invention to provide an improved Hall sensor array which is also less complicated to manufacture.
This object is achieved by a Hall sensor array comprising:
a first and at least one additional pair of Hall sensor elements,
wherein each Hall sensor element has four terminals, of which two terminals act as power supply terminals for supplying an operating current and two terminals act as measurement terminals for measuring a Hall voltage,
wherein the Hall sensor elements are so arranged that the current directions of the operating current in the two Hall sensor elements of each pair are offset at an angle of approximately 90xc2x0 to one another,
wherein the Hall sensor elements of the additional pair(s) are so arranged that their current directions of the operating current are offset at an angle of approximately 90xc2x0/n to the current directions of the operating current of the first pair of Hall sensor elements, n being the total number of Hall sensor element pairs, and
wherein respective first terminals of the measurement terminals of the Hall sensor elements and respective second terminals of the measurement terminals of the Hall sensor elements are connected together for measurement of the Hall voltage,
wherein the Hall sensor array also has switches and wherein the respective terminals of the Hall sensor elements are connected to the switches, so that the respective first and second supply terminals for supplying an operating current and the respective first and second measurement terminals for measuring a Hall voltage can be switched over from one measurement to a subsequent measurement in such a way that the current directions of the operating current in the Hall sensor elements and the Hall voltage tapping directions can be rotated through approximately 90xc2x0 from one measurement to a subsequent measurement,
wherein the Hall sensor array also has a controller by means of which the switches are controllable in such a way that the Hall sensor array is operable in spinning current operation for generating a Hall signal and wherein the offset voltages of the Hall sensor elements approximately cancel one another out in a revolution so that the Hall signal contributions which actually depend on the magnetic field remain, and
wherein respective first supply terminals of each Hall sensor element are connected together and to a first terminal of a common voltage source and respective second supply terminals of each Hall sensor element are connected together and to the second terminal of the common voltage source so that the common voltage source supplies an operating current for the Hall sensor elements.
The present invention is based on the finding that the sensor signal offset contribution in a Hall sensor array can be greatly reduced through the above geometrical arrangement of the individual Hall sensor elements of the pairs and the interwiring of the terminals, making it possible to supply an offset signal which is already precompensated. With the arrangement and interwiring of the Hall sensor elements according to the present invention it is possible to make the measured Hall voltage independent of the crystal direction of the semiconductor material.
According to the present invention, the Hall sensor elements are operated in the so-called xe2x80x9cspinning currentxe2x80x9d mode. In spinning current operation the measurement direction is rotated continuously in a cycle by e.g. 90xc2x0 at a particular clock frequency, i.e. the operating current flows from one electrode to the facing contact electrode, the Hall voltage being tapped off at the transverse contact electrodes, whereupon the measurement direction is rotated through 90xc2x0 at the next cycle, i.e. the next measurement phase. The Hall voltages measured in the individual measurement phases are evaluated by a suitable correctly signed and weighted summation or subtraction. The offset still contained in the individual measurement phases can be reduced still further or the offset voltages during a revolution should roughly cancel one another out, so that the parts of the Hall signal which really depend on the magnetic field are retained.
Because of the orientation and interwiring of the Hall sensor elements according to the present invention it is no longer necessary to take the crystal direction of the semiconductor material into account, whereby the influence on the measured Hall voltage of the piezo effects dependent on the crystal direction of the semiconductor material can effectively be eliminated completely.
Since the strong dependence of the offset signal both on the crystal direction of the semiconductor material and on the inhomogeneties and irregularities in the semiconductor material is effectively eliminated by the Hall sensor array according to the present invention, a considerable increase in the sensitivity and measurement accuracy is achieved by means of this Hall sensor array. This permits the complexity of the circuitry needed for correct evaluation and further processing of the sensor signals to be reduced in the Hall sensor array according to the present invention.
The improved sensitivity and measurement accuracy resulting from the lower offset contribution of the sensor signal of the Hall sensor array also results in an increase in the exploitable resolution of the measured Hall voltage.
A further advantage of the present invention is that the circuit complexity of the Hall sensor array can be reduced further since the fixed interwiring of the Hall sensor elements makes it possible to apply a common operating current to all the Hall sensor elements and to have a common tap-off for all the Hall signals of the Hall sensor elements. This avoids the need to supply a separate operating current to each sensor element individually and to measure separately the Hall voltage of each sensor element. This avoids the need for additional circuit components, e.g. additional switches, current sources, feed lines, etc. Furthermore, the complexity of the evaluation circuit can be simplified.